It is known to use, in some parts of the tires, rubber compositions exhibiting a high stiffness during small deformations of the tire (cf. WO 02/10269). The resistance to small deformations is one of the properties which a tire has to exhibit in order to respond to the stresses to which it is subjected.
This stiffening can be obtained by increasing the level of reinforcing filler or by incorporating certain reinforcing resins in the constituent rubber compositions of the parts of the tire.
However, in a known way, the increase in the stiffness of a rubber composition by increasing the level of filler can be disadvantageous to the hysteresis properties and thus rolling resistance properties of the tires. In point of fact, it is an ongoing aim to lower the rolling resistance of tires in order to reduce the consumption of fuel and thus to protect the environment.
Conventionally, this increase in the stiffness is obtained by incorporating reinforcing resins based on a methylene acceptor/donor system. The terms “methylene acceptor” and “methylene donor” are well known to a person skilled in the art and are widely used to denote compounds capable of reacting together to generate, by condensation, a three-dimensional reinforcing resin which will become superimposed and interpenetrated with the reinforcing filler/elastomer network, on the one hand, and with the elastomer/sulphur network, on the other hand (if the crosslinking agent is sulphur). The methylene acceptor described above is combined with a curing agent, capable of crosslinking or curing it, also commonly known as “methylene donor”. Crosslinking of the resin is then brought about during the curing of the rubber matrix by formation of bridges (—CH2—) between the carbons in the ortho and para positions of the phenolic nuclei of the resin and the methylene donor, thus creating a three-dimensional resin network.
The methylene donors conventionally used in rubber compositions for tires are hexamethylenetetramine (abbreviated to HMT) or hexamethoxymethylmelamine (abbreviated to HMMM or H3M) or hexaethoxymethylmelamine.
However, the combination of phenolic resin conventionally used, which is a methylene acceptor, with HMT or H3M, which is a methylene donor, produces formaldehyde during the vulcanization of the rubber composition. In point of fact, it is desirable to reduce, indeed even in the long run to eliminate, the formaldehyde of rubber compositions due to the environmental impact of this compound.
Unexpectedly, applicants have discovered, during their research, that polyaldehydes can advantageously replace conventional methylene donors while preventing the production of formaldehyde. The use of these polyaldehyde compounds makes it possible not only to obtain rubber compositions exhibiting the same low-strain stiffness as the conventional rubber compositions which comprise HMT or H3M methylene donors but, surprisingly, to greatly improve the fatigue strength of the rubber compositions and thus the endurance of the tires.